Subsequent Events (Details) (USD $)	0 Months Ended					12 Months Ended	0 Months Ended		6 Months Ended
	Feb. 14, 2014	Nov. 14, 2013	Aug. 14, 2013	May 15, 2013	Feb. 14, 2013	Dec. 31, 2013	May 15, 2014 Scenario, Forecast		Jun. 30, 2014 Scenario, Forecast	May 06, 2014 Subsequent Event [Member] Commercial Paper [Member]
Subsequent Event [Line Items]
Borrowing capacity										$ 1,000,000,000.0
Cash distribution per unit	$ 0.585	$ 0.5575	$ 0.5325	$ 0.50750	$ 0.5000	$ 2.0975	$ 0.6125	[1]	$ 1.1975
Total cash distributions	$ 132,835,000	$ 126,374,000	$ 120,707,000	$ 115,040,000	$ 113,340,000	$ 475,461,000	$ 139,079,000	[1]	$ 271,914,000

[1] Our general partner's board of directors declared this cash distribution on April 24, 2014 to be paid on May 15, 2014 to unitholders of record at the close of business on May 8, 2014.